Various 9-substituted purines are known as anti-viral and anti-neoplastic agents. One such compound, known as AZT, has been used for the treatment of AIDS. More recent examples are disclosed in EP-A-4268672, U.S. Pat. No. 4,742,064, and also in GB-A-2217320 which, by way of specific illustration, describes a compound known as Carbovir (carbocyclic 2',3'-didehydro-2',3'-dideoxyguanosine). Not surprisingly, the enantiomers of such chiral compounds have different activities.
Carbovir and known analogues are prepared from the known .gamma.-lactam, 2-azabicyclo[2.2.1]hept-5-en-3-one, i.e. the compound of formula ##STR1## in which X is --CH.sub.2 --, --Y--Z-- is --CH.dbd.CH--, and R.sub.n is absent. The prior art indicates that the final product, or any intermediate or starting material, may be resolved by known methods, and that a racemic mixture of the product may be enzymatically converted to chirally pure compounds. The .gamma.-lactam can be prepared by reacting cyclopentadiene with tosyl cyanide.
R. D. Allan et al, Eur. J. Pharmacol. 122 (1986) 339-348, disclose a series of GABA analogues, resolved and unresolved, including compounds of formula II ##STR2## in which --X-- is --CH.sub.2 --, --Y--Z-- is --CH.dbd.CH--, and R.sub.1 and R.sub.2 are H. In particular, (+)-1R,4R-4-aminocyclopent-2-ene-1-carboxylic acid is disclosed, although its activity is much lower than that of the structurally-isomeric 1-ene.